1. Field of the Invention
The present invention relates to a field effect transistor (FET), and more particularly to a metal semiconductor field effect transistor (MESFET).
2. Description of the Related Art
In recent years, mobile communication devices such as car telephones have spread, and direct broadcasting by satellite and commercial satellite communications are widely utilized. Since such communications adopt radio waves of high frequencies, a high frequency receiver with low noise and good distortion characteristics is strongly required. Therefore, a MESFET using a compound semiconductor which has a superior high frequency characteristic, or a high electron mobility transistor (HEMT) which is one type of the MESFETs is used in a high frequency receiver.
In such a transistor, generally, by reducing the gate length thereof, the high frequency characteristic is improved. However, the reduced gate length causes a drain conductance to increase. Moreover, in the saturation area of a drain current, the drain conductance is increased in a nonlinear manner. As a result, there arise a problem in that the distortion characteristic is deteriorated. There also arise a problem in that the gain and noise figure cannot be improved because of the influence by the drain conductance which is increased by the reduced gate length.
In order to solve these problems, a method has been proposed in which a channel region of a MESFET is patterned so as to be a stripe shape, so that carriers contributing to the conduction are confined in one-dimension, whereby a transconductance (gm) is improved and the drain conductance is prevented from being increased in a nonlinear manner.
FIG. 31A is a plan view showing a HEMT having a stripe-shaped channel according to the above method (hereinafter, such a HEMT is referred to as a stripe-channel HEMT). FIG. 31B is a cross-sectional view taken along line 31B--31B in FIG. 31A. A structure of the stripe-channel HEMT is described with reference to these figures. On 8 semi-insulating GaAs substrate 101, a GaAs layer to which impurities are not doped (an i-GaAs layer) 102 is formed. On the i-GaAs layer 102, an i-AlGaAs layer 103 is formed. On the i-AlGaAs layer 103, an n-type AlGaAs layer (an n-AlGaAs layer) 104 is formed. By forming e plurality of recesses 111 which reach the i-GaAs layer 102 from the surface of the n-AlGaAs layer 104, a plurality of parallel ridges 110 are formed. At both ends of each of the plurality of ridges 110, a source electrode 210 and a drain electrode 211 are formed on the n-AlGaAs layer 104. A gate electrode 201 is formed on top and side faces of the ridges 110 and bottom faces of the recesses 111. A two-dimensional electron gas layer 170 is formed in the i-GaAs layer 102 in the vicinity of an interface between the i-GaAs layer 102 and the i-AlGaAs layer 103.
FIG. 32 is an energy band diagram of the stripe-channel HEMT. In FIG. 32, E.sub.c and E.sub.v represent a conduction band and a valence band respectively, and E.sub.f represents the Fermi level. The symbol "=" on the gate metal side represents an interface state, and the symbol "+" in the n-AlGaAs layer 104 represents a space charge. In the HEMT which is practically fabricated, the i-AlGaAs layer 103 is interposed between the i-GaAs layer 102.and the n-AlGaAs layer 104 so as to sufficiently spatially separate electrons from ionized impurities. However, the i-AlGaAs layer 103 is not shown in FIG. 32.
As is shown in FIG. 32, impurities doped to the n-AlGaAs layer 104 are ionized, and electrons are separated. The separated electrons are drawn to the i-GaAs layer 102. In detail, since the separated ions are restricted by positive charges of impurity ions remaining in the n-AlGaAS layer 104, the separated electrons are drawn into a portion of the i-GaAs layer 102 in the vicinity of the interface between the i-GaAs layer 102 and the n-AlGaAs layer 104. Such a portion is referred to as a two-dimensional electron gas layer 170. The two-dimensional electron gas layer 170 only has a thickness of several nanometers (nm). This means that the two-dimensional electron gas layer 170 is substantially formed in two-dimension. Accordingly, the separated electrons can be moved in a two-dimensional electron gas layer 170 in a two-dimensional direction. Such gas is referred to as a two-dimensional electron gas.
A HEMT is a transistor which uses the two-dimensional electron gas as a carrier. Since the two-dimensional electron gas is moved in the i-GaAs layer 102 which has extremely low impurity concentration, the movability of the electrons is extremely high. As a result, it is possible for the HEMT to obtain much higher gm as compared with the usual MESFET. Moreover, since the electrons are not scattered by impurities, the HEMT is superior in noise characteristics.
The two-dimensional electron gas is controlled by changing the gate potential. In general, a drain conductance is increased in a non-linear manner, as the drain voltage is increased. In order to solve the problem, the channel portion is made narrower and hence the electrons are in a quasi one-dimensional state, whereby the confining effect of the electrons is improved. Thus, the gm is improved, and the drain conductance is prevented from being increased in the non-linear manner. In the stripe-channel HEMT shown in FIG. 31A, a channel width L.sub.ch is 0.2-0.3 .mu.m. The two-dimensional electron gas layer 170 is made narrower by the ridges 110.
FIG. 31C is a cross-sectional view taken along line 31C--31C' in FIG. 31A. FIG. 31D is a plan view of the two-dimensional electron gas layer 170. Below the source electrode 210 and the drain electrode 211, a first channel region 171 and a second channel region 172 are formed respectively, The first channel region 171 and the second channel region 172 form ohmic junctions with the source electrode 210 and the drain electrode 211 respectively. By forming the ridges 110, a plurality of stripe-like middle channel regions 176 are formed. The electrons supplied from the source electrode 210 flow to the drain electrode 211 via the first channel region 171, the plurality of stripe-like middle channel regions 176 and the second channel region 172.
As described above, in the stripe-like middle channel regions 176, the channels are narrowed, so that the electrons are confined substantially in the two directions. That is, the electrons have mobility only in one direction. The electrons in this condition have higher mobility than the two-dimensional electrons.
Referring to FIGS. 33a to 33C, a method of fabricating a conventional stripe-channel HEMT is briefly described. On a semi-insulating GaAs substrate 101, an i-GaAs layer 102, an i-AlGaAs layer 103, and an n-AlGaAs layer 104 are successively formed. Thereafter, a resist pattern 400 is formed, and the semiconductor layers are etched so as to reach the semi-insulating GaAs substrate 101 for isolating elements from each other. Though not shown, a source electrode and a drain electrode made of a gold-germanium alloy and nickel are formed on the n-AlGaAs layer 104. Next, a resist pattern 401 is formed, and the semiconductor layers are etched so as to form a plurality of ridges 110. After removing the resist pattern 400, a resist pattern which defines a gate electrode pattern having a width of 0.2-0.3 .mu.m is formed by an electron beam exposure method or a phase shift method. Then, titanium and aluminum are deposited, and a gate electrode 201 is formed by a lift-off technique.
FIGS. 34A and 34B show the V.sub.gs -gm characteristic and the V.sub.ds -I.sub.ds characteristic of the conventional stripe-channel HEMT and the conventional MESFET, respectively. As compared with the conventional MESFET, the transconductance (gm) of the conventional stripe-channel HEMT is improved by 20% or more. Also, in the conventional MESFET, the I.sub.ds is substantially constant in the vicinity of V.sub.ds -3 V, and the drain conductance characteristic is improved.
However, the conventional stripe-channel MESFET has the following problems:
(1) the striped channel causes the source resistance to increase, so that it is difficult to improve the transconductance; PA0 (2) the gates are formed in regions other than the channel regions of the FETs, so that the gate capacitance is increased and the noise figure is degraded; PA0 (3) when a HEMT structure is adopted, the two-dimensional electron gas is in contact with the gate electrode, so that the gate breakdown voltage is lowered and the gate leakage current is increased; and PA0 (4) instead of the striped channel, the electron accumulation layer is enlarged as the drain voltage rises, so that it is impossible to prevent the drain conductance from being increased in the non-linear manner. PA0 (1) The source and drain resistances are decreased and the drain conductance can be prevented from being increased in a non-linear manner while the drain-source voltage rises, so that the transconductance is improved and superior gain and noise characteristic can be obtained. PA0 (2) By forming gate electrodes only in the plurality of striped channel regions of the FETs so as to connect the gate electrodes via low-resistance interconnections, the gate capacitance and the gate resistance can be reduced, so that a superior noise characteristic can be obtained. PA0 (3) A channel is made into a one-dimensional form due to a depletion layer at the boundary between a p-type impurity layer and an n-type impurity layer, and due to a depletion layer formed by a gate Schottky junction, so that the drain conductance is prevented from being increased in a non-linear manner, whereby a superior third-order distortion characteristic can be obtained. PA0 (4) By additionally providing voltage applying means for the p-type impurity layer, and by applying reverse bias voltage between the n-type impurity layer and the p-type impurity layer, the channel width is further reduced, and the one-dimensional effect can be enhanced, whereby a more superior third-order distortion characteristic can be obtained. PA0 (5) Due to a double heterostructure in which a channel layer having a narrow energy gap is sandwiched between a first and a second semiconductor layers each of which has a wide energy gap, electrons can be confined in the channel layer, so that the one-dimensional effect can be enhanced, whereby a superior third-order distortion characteristic can be obtained. PA0 (6) Due to a double heterostructure in which a channel layer having a narrow energy gap is sandwiched between a first and a second semiconductor layers each of which has a wide energy gap, electrons can be confined in the channel layer; and by providing voltage applying means for the first semiconductor layer, a barrier between the channel layer and the first semiconductor layer is enhanced, so that the one-dimensional effect can be further enhanced, whereby a superior third-order distortion characteristic can be obtained. PA0 (7) Due to a double heterostructure in which a channel layer having a narrow energy gap is sandwiched between a first and a second semiconductor layers each of which has a wide energy gap, electrons can be confined in the channel layer; and by selectively etching the channel layer so as to make the length of a stripe channel portion of the channel layer in the gate length direction smaller than that of the stripe channel portion of the second semiconductor layer in the gate length direction, so that the one-dimensional effect can be enhanced, whereby a superior third-order distortion characteristic can be obtained. Moreover, the stripe channel portion of the channel layer can be prevented from being in contact with e gate electrode, whereby the occurrence of the gate leakage current can be suppressed. PA0 (8) In a HEMT structure, by making the length of a stripe channel portion of an electron supplying layer in a gate length direction smaller than that of a stripe channel portion of a channel layer in the gate length direction, a two-dimensional electron gas layer is prevented from being in contact with a gate metal, the gate leakage current can be prevented from occurring, and the length of the two-dimensional electron gas layer in the gate length direction can be made smaller. As a result, the one-dimensional effect can be enhanced, whereby a superior Schottky characteristic and a superior third-order distortion characteristic can be obtained. PA0 (9) Due to the electron confining effect by a quasi one-dimensional channel, and due to a multi-gate transistor, the drain conductance is prevented from being increased in a non-linear manner, whereby a superior third-order distortion characteristic can be obtained.